When the human body plunges ­into the January waters of the Massachusetts coast, as more and more people do each New Year’s Day, it will race through all sorts of painful physical stages in the first minute. Surviving that pain, those who have come out of the water say, is the point. And, maybe, the pleasure.

The ritual of dunking oneself in icy water is ancient, found throughout the world’s cultures, and has been going on for millennia in Massachusetts, courtesy of the Wampanoag, who are thought to have participated in dousing rituals as far back as 5000 BC.

But the New Year’s Day group plunge, which began in North America in 1901 with the L Street Brownies in South Boston, has boomed in popularity, with more and more organized swims across the country. On Tuesday, groups large and small will hit the icy water from ­Marblehead and Swampscott to Tinean Beach in Dorchester, down to Hull and Hingham and all along Cape Cod.

Jumping into frigid water, however, goes against human instinct, and doctors say there is good reason for that.

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During that first contact with the water, as the rapid cooling begins, the body goes through something called the cold shock response, an unpleasant mix that can include gasping or even hyperventilating. In worst-case scenarios, the rush of adrenaline and other hormones could trigger a cardiac episode, said Alasdair Conn, chief of emergency medicine at Massachusetts General Hospital. Conn advises those with a known cardiac condition not to take the plunge.

Cold shock response lasts about a minute. This is important to know, mentally, because it is a seriously painful minute. Hypothermia won’t happen for several minutes, as many as 30, depending on the person, say specialists, though the extremities can numb quickly as the body tries to protect the core.

“It’s a dare thing, a way to dare your friends and dare yourself,” said Jack Dever, who oversees the modern Brownies, a hundred or so hard-core winter swimmers who swear by the health benefits of cold water. The 73-year-old said he swims a few times a week, taking a couple of dozen strokes before heading back into the sauna. On Jan. 1, he and the Brownies are joined by a few hundred others on the beach at Curley Community Center.

Dever said the increasing popularity of cold-water swims has much to do with the uptick in charitable events. The chilly plunge has joined such things as the charity road race as the motivator to entice people to put their bodies through new extremes, with a few quick seconds of courage replacing months of training.

“There’s a lot of screaming when you first go in,” said Scott Burnham, who organized a group plunge at Good Harbor Beach Saturday that raised ­almost $100,000 to benefit an ALS patient named Pete Frates, a 27-year-old former captain of the Boston College baseball team.

“It was just instant freeze,” Burnham said. “I’m 6 foot 4, so I had to go about 100 yards to get deep enough to jump under, and after that, running back to the beach trying to find my wife was probably the biggest panic moment of my life. The water is warmer than the air, so getting out is miserable.”

Once he was warm in his towel, he vowed, as many do, that he would never do it again.

Heather Foley, a humor blogger for a South Boston website, made her first and only plunge with the Brownies two years ago and said the shock was so miserable she wanted to cry.

“I was not prepared,” she said. “I got up to my knee, and I immediately tried to leave, but I was one of the first people in, so it was like trying to flee a herd of wildebeest.” She ultimately had to go deeper before she could make a right turn and get around the crowd. “Your body wasn’t made to do that, and anyone who says otherwise lies. I didn’t dunk my head in, so people said it didn’t count. Sorry. I’m not doing it again.”

Some report a high from the experience, similar to runner’s high. Swimmers in other cultures have claimed it as a form of asceticism. Porfiry Ivanov, a Russian mystic, preached the health benefits of a cold weather cleansing.

The key to a successful modern dip, Dever says all the regular Brownies know, is something that is often forgotten on New Year’s Eve: moderation. In and out.

The body will tolerate a quick dip, Dever said.

It is the mind that is the real enemy.

“If I talk to somebody who’s never done it, I’ll just say go blank, put it out of your mind,” he said. “If you stop and think, you’re not going to go in.”

Fred Ahearn, director of the Curley Community Center, who will announce the countdown to the 10:30 a.m. plunge on his megaphone, said there is an upside to the pain: It will make you forget about a hangover.

“They go into the water a little blurry-eyed from the night before, but they come out fired up and ready to go and have a couple more beers,” he said.